Exact Average OFDM Subcarrier SINR Analysis Under Joint Transmit-Receive I/Q Imbalance

The subcarrier signal to interference-plus-noise ratio expression for orthogonal frequency division multiplexing systems impaired by in-phase/quadrature (I/Q) imbalance is a ratio of correlated random variables. In its original form, a quadruple integral is required to evaluate the exact average subcarrier SINR which is cumbersome to compute numerically. We show that the quadruple integral can be reduced to a single integral which can be easily evaluated numerically. Furthermore, closedform expressions are derived for several important special cases including TX-only I/Q imbalance, RX-only I/Q imbalance and joint TX/RX I/Q imbalance with equal levels of I/Q imbalance at the transmitter and at the receiver. In addition, we derive high input SNR approximations which provide valuable insights on how the average SINR is affected by the I/Q imbalance levels at the transmitter and the receiver and by the channel correlation between the image OFDM subcarriers. For example, we show that at high input-SNR, TX-only I/Q imbalance is more harmful, in terms of average subcarrier SINR, than RX-only I/Q imbalance. Furthermore, we show that an approximate OFDM subcarrier SINR expression, commonly used in the literature, is not accurate when RX I/Q imbalance is present. Keywords—I/Q Imbalance, SINR, OFDM